Variation in the mating system of Vincetoxicum hirundinaria (Asclepiadaceae) in peripherial island populations

BACKGROUND AND AIMS: Self-fertility may be selected for in small and isolated plant populations of normally outcrossing species. In addition, adaptations for self-fertility are likely to arise in island populations and in populations that are located at the border of the species range. The mating system of Vincetoxicum hirundinaria (Asclepiadaceae) is examined in island populations that are located at the northern border of the species range. METHODS: Pollination experiments were conducted under glasshouse conditions with plants from four populations. KEY RESULTS: The frequencies of self-fertile individuals were relatively high and did not differ among populations. Cross-pollination resulted in higher fruit set than self-pollination. However, fruit-set from self-pollination and cross-pollination did not differ in the self-fertile individuals. Interestingly, the proportion of aborted fruits was on average higher following cross-pollination than following self-pollination. No differences were observed in seed number or seed mass between self- and cross-pollinated fruits. Pollen tube growth following self- and cross-pollinations was indistinguishable. CONCLUSIONS: The results of this study revealed that V. hirundinaria possess a mixed-mating system in the studied island populations. Evidence was also provided for a late-acting self-incompatibility system commonly observed in Asclepiadaceae. No clear signs of inbreeding depression were observed in the early stages of development.     

Female reproductive success decreases with display size in monkshood, Aconitum kusnezoffii (Ranunculaceae)

Background and Aims

Reduction in female fitness in large clones can occur as a result of increased geitonogamous self-fertilization and its influence through inbreeding depression. This possibility was investigated in the self-compatible, bee-pollinated perennial herb Aconitum kusnezoffii which varies in clone size.

Methods

Field investigations were conducted on pollinator behaviour, flowering phenology and variation in seed set. The effects of self-pollination following controlled self- and cross-pollination were also examined. Selfing rates of differently sized clones were assessed using allozyme markers.

Key Results

High rates of geitonogamous pollination were associated with large display size. Female fitness at the ramet level decreased with clone size. Fruit and seed set under cross-pollination were significantly higher than those under self-pollination. The pre-dispersal inbreeding depression was estimated as 0·502 based on the difference in seed set per flower between self- and cross-pollinated flowers. Selfing rates of differently sized clones did not differ.

Conclusions

It is concluded that in A. kusnezoffii the negative effects of self-pollination causing reduced female fertility with clone size arise primarily from a strong early-acting inbreeding depression leading to the abortion of selfed embryos prior to seed maturation.Key words: Early-acting inbreeding depression, Aconitum kusnezoffii, clone size, female reproductive success, geitonogamy     

Effects of self-pollination and outcrossing with cultivated plants in small natural populations of American ginseng, Panax quinquefolius (Araliaceae)

For rare plants, self-pollination and inbreeding can increase in small populations, while unusual levels of outcrossing can occur through restoration efforts. To study both inbreeding and outcrossing, we performed experimental pollinations using Panax quinquefolius (American ginseng), a wild-harvested plant with a mixed mating system. For inbreeding, plants were either cross-pollinated within the population or self-pollinated, which resulted in a higher proportion of seeds from self-pollinated flowers. For outcrossing, wild plants were either cross-pollinated within the population or with cultivated plants from West Virginia or Wisconsin. Offspring of all crosses were followed for 4 yr. Two-yr-old seedlings from self-pollination had 45% smaller leaf areas and 33% smaller heights relative to those from cross-pollination. Leaf area is a positive predictor of longer-term survival in wild populations. Our results suggest inbreeding depression, which is unexpected in this self-fertile species. Seedlings from crosses with cultivated plants had 127% greater leaf area and 165% greater root biomass relative to outcrosses within the population. The accelerated growth suggests genetic differences between wild and cultivated populations, but outbreeding depression may not appear until later generations. Assessment of the ultimate fitness consequences of introducing cultivated genotypes requires monitoring over longer time periods.     

Ovule pre-emption and pollen limitation in a self-fertile perennial herb (Blandfordia grandiflora,Liliaceae)

The extent, frequency and causes of pollenlimited seed production were examined in partially selffertile populations of Blandfordia grandiflora for 2 years. Percentage seed set of open-pollinated plants (50–57%) did not differ within or between years, and was about 19% less than experimentally cross-pollinated plants (70–75%). Floral visits by honeybees did not differ through the flowering season and the number of pollen grains deposited on stigmas within 12 h of flowers opening exceeded the number of ovules per flower, indicating that the quality rather than the quantity of pollination limited seed set. Pollen limitation was caused by concurrent self- and cross-pollination and the subsequent abortion of some selfed ovules due to inbreeding depression. Natural seed set (55%) was intermediate between selfed (43%) and crossed (75%) flowers and was not increased when flowers that had been available to pollinators for 24 h were hand cross-pollinated, suggesting that ovules were already fertilized. Similarly, experimental pollination with both cross and self pollen within 24 h of flowers opening did not increase seed set relative to natural seed set, indicating that both cross- and self-fertilizations had occurred. In contrast, when selfing followed crossing by 48 h, or vice versa, seed set did not differ from crossed-only or selfed-only flowers, respectively, indicating that ovules were pre-empted by the first pollination. Collectively, these results indicate that under natural conditions self pollen pre-empts ovules, rendering them unavailable for cross-fertilization. This selfing reduces fecundity by 50%, as estimated from the natural production of cross seeds when selfing was prevented. Consequently, selection should favour floral traits, such as increased stigma-anther separation or protandry, that reduce interference between male and female functions that leads to selfing.     

Pollen quality limits seed set in Burchardia umbellata (Colchicaceae)

In self-incompatible plants, interference by self pollen or genetically related pollen can potentially exacerbate pollen limitation, although this has rarely been demonstrated. We examined the breeding system, pollen limitation, and pollen interference using self- and cross- pollinations and pollen supplementations in Burchardia umbellata, an insect-pollinated lilioid monocot. Ovule fertilization and seed set were less following selfing than crossing (22 vs. 78% and 4 vs. 73%, respectively), indicating partial self-incompatibility. Flowers were partially protandrous, and flowers opened concurrently on plants potentially allowing self pollen interference. Natural seed set was pollen limited and varied within and among years, probably due to variation in flowering plant density. Interference by self or genetically related pollen caused pollen limitation as evidenced by increased seed set of bagged cross-pollinated plants compared to unbagged pollen-supplemented plants in two years. In 1996, both fertilization and seed set increased in response to cross-pollination, indicating that interference occurred in the style and ovary. In 1997, only seed set increased after cross-pollination indicating that interference occurred in the ovary. Inappropriate pollen deposition may contribute to pollen limitation more often than previously recognized and should select for floral traits that decrease deposition of self or related pollen.     

HOW DEPRESSED? ESTIMATES OF INBREEDING EFFECTS DURING SEED DEVELOPMENT DEPEND ON REPRODUCTIVE CONDITIONS

Inbreeding depression can reduce the performance of offspring produced by mating between relatives, with consequences for population dynamics and sexual-system evolution. In flowering plants, inbreeding depression commonly acts most intensely during seed development. This predispersal component is typically estimated by comparing seed production following exclusive self- and cross-pollination, but such estimates are unbiased only if seed production is limited by ovule availability, rather than by pollen receipt or seed-development resources. To overcome this problem, we propose experimental and statistical methods based on a model of ovule fertilization and seed development that accounts for differential fertilization by self- and cross-pollen, limited ovule viability or receptivity, differential survival of self- and cross-zygotes and limited resource availability. Simulations illustrate that the proposed methods eliminate bias in estimated predispersal inbreeding depression caused by pollen limitation and can improve estimates under resource limitation. Application of these methods to two orchid species further demonstrates their utility in identifying and estimating diverse influences on reproductive performance under typical conditions. Although our theoretical results raise questions about the reported intensity of predispersal inbreeding depression, our proposed methods guard against bias while also providing insight into plant reproduction.     

Pollination biology in an endangered rocky mountain toadflax (Linaria cavanillesii)

Knowledge about the reproductive system of species inhabiting rocky habitats is scarce. The reproductive biology (floral biology, experimental pollination, insect visits, inbreeding depression, and seed predation) of the rupicolous Linaria cavanillesii was analyzed under field and experimental conditions. Self-compatibility was revealed by the high fruit set, seed set, and seed mass in the pollination experiment. Furthermore, results disclose that this species does not need insect visitors for seed production since fruit set after autonomous self-pollination was similar to that by hand cross-pollination. Self-offsprings were not affected by a strong degree of inbreeding depression in early life-cycle stages. Seed predation by Cucurlionidae is the main limiting factor of fruit and seed production in this species. Contrary to other perennial species of Linaria previously studied, L. cavanillesii represents the first clear report of self-fertility.     

Early Inbreeding Depression and Pollen Competition inCalluna vulgaris(L.) Hull.

We investigated whether partial self-sterility inCalluna vulgarisresultsfrom abortion of selfed offspring owing to inbreeding depressionor a late-acting self-incompatibility mechanism, and whetherself-pollen interferes with normal functioning of cross-pollen.Self-pollination resulted in 75% less seed set than cross-pollination.Self-pollen tubes reached ovaries and penetrated ovules as oftenas those of cross-pollen. Following self-pollination, examinationof the size of undeveloped seeds showed that at least 70% resultedfrom ovule fertilization and arrest of development occurredat various stages. All self-pollinated plants produced seedsand self-fertility varied among plants. These results indicatethat the reduced seed set observed in self-pollination is morelikely the result of inbreeding depression rather than a late-actingself-incompatibility system. The fecundity component of inbreedingdepression was high (0.762). Seed set was reduced by an averageof 40% when self-pollen was mixed with cross-pollen, comparedto pure cross-pollination. Using genetic markers, we found about20% of seeds resulted from self-pollination in mixed-pollinatedfruits.C. vulgarisis likely to experience self-pollination innature and our data suggest this will reduce the number of ovulesthat might otherwise mature after cross-pollination.Copyright1999 Annals of Botany Company Calluna vulgaris(heather), self-pollination, pollen tube, ovule fertilization, early inbreeding depression, pollen interference.     

Elevation-related variation in the population characteristics of distylous Primula nivalis affects female fitness and inbreeding depression

The population characteristics of distylous species are highly sensitive to stochastic natural selection pressure.Therefore,populations growing under different environmental conditions may vary in floral morph ratios,potentially affecting female fitness and leading to inbreeding depression.However,the variation in offspring quality among populations as a result of inbreeding depression is poorly understood in distylous species.This study investigates variations in plant density,seed mass,seed viabilityfemale fitness,and post-dispersal inbreeding depression in both sexual morphs(long-styled and shortstyled plants)of the distylous Primula nivalis that were subjected to different pollination treatments along an elevational gradient from 1657 to 2704 m a.s.l.Population characteristics(morph plant density and ratio)and fruit set were significantly affected by sexual morph and elevation.Plant density and fruitset frequencies were lower for short-styled than for long-styled plants at 2704 m a.s.l.The seeds from the cross-pollinated flowers of both morphs were higher in quality than those of self-pollinated flowers.The female fitness of seeds from cross-pollinated flowers of both morphs was higher than that of seeds from open-pollinated and self-pollinated flowers.The female fitness of seeds from long-styled flowers was higher than that of seeds from short-styled flowers at all elevations.Inbreeding depression increased with elevation among plants with short-styled flowers but not among those with long-styled flowers.Variation in the elevation-dependent mating system might influence female fitness and affect inbreeding depression in both floral morphs.In conclusion,the low quality of seeds from short-styled flowers at high elevations might decrease short-styled flower frequency,affecting population characteristics.     

Fertility differences among floral morphs following selfing in tristylous Eichhornia paniculata (Pontederiaceae): inbreeding depression or partial incompatibility?

Reduction in seed set following self- vs. cross-pollination in flowering plants can result from abortion of selfed offspring owing to inbreeding depression and/or partial self-incompatibility. Previous studies on tristylous Eichhornia paniculata (Pontederiaceae) indicate that reduced seed set following self-pollination generally occurs in the short- (S), but not the long-(L) or mid-styled (M) morphs. To determine whether this pattern results from morph-specific differences in inbreeding depression owing to the sheltering of deleterious alleles at the S locus and/or partial self-incompatibility, we conducted controlled hand-pollinations of the floral morphs and measured seed set and levels of seed abortion. There were no significant differences in fertilization success and seed set following self-, illegitimate, and legitimate pollinations in the L and M morphs. In contrast, in the S morph self-, intramorph and intermorph illegitimate pollinations resulted in significant reduction in seed set in comparison with legitimate pollination. This indicates that the reduced seed set observed in self-pollination is the result of partial incompatibility rather than inbreeding depression. Significantly reduced fertilization success and low levels of ovule abortion in illegitimate pollinations of S plants also supported this conclusion. Reduced fertility in the S morph may have implications for the observed loss of this morph from natural populations and the evolutionary breakdown of tristyly.     

The cost of inbreeding in Platanthera leucophaea (Orchidaceae)

Fragmentation and isolation are expected to have a considerable impact on viability and recruitment in populations of rare species. Platanthera leucophaea (Orchidaceae), a rare orchid, currently exists in a fragmented landscape of its natural habitat. Floral morphology suggests this species is predominantly outcrossing, but surveys of allozyme diversity suggest high, variable levels of inbreeding in populations (F(IS) = -0.078 to 1.0). This study examines the potential cost of inbreeding and the extent to which inbreeding depression can vary temporally and in populations of different size and genetic structure. Flowers were pollinated by hand in one large population and one small population over three seasons. Seed set, seed mass, and seed viability were compared among self-, outcross-, and open-pollinated fruits. Seed set was greater than 50% in both populations for all years of study. High levels of inbreeding depression were detected in seed viability but not in seed mass in both populations. However, the magnitude of inbreeding depression differed over years and between populations, a pattern that reflects differing environmental conditions and variable evolutionary and demographic histories. Consequently, conservation of this species will be most successful if outcrossing is promoted in populations by maximizing population size and genetic variability.     

Effects of inbreeding on traits that influence dispersal and progeny density in Cakile edentula var. lacustris (Brassicaceae)

Inbreeding may influence the intensity of sibling competition by altering the number of offspring produced or by changing plant morphology in ways that influence seed dispersion patterns. To test this possibility, effects of inbreeding on seed production and on traits that influence progeny density were measured using experimental pollinations of flowers of Cakile edentula var. lacustris. Different flowers on a plant were either hand pollinated with self pollen (with and without emasculation) or foreign pollen, or they were allowed to be pollinated naturally. Selfed flowers matured significantly fewer viable seeds than outcrossed flowers (10.3% less seed maturation with inbreeding depression of 19.2%), due in large part to a greater percentage of proximal seed abortions and lower germination success. Plants grown from selfed seeds tended to have lower seed production (37 fewer seeds on average, with inbreeding depression of 16.2%), caused in part by an increase in the percentage of fruits with proximal seed abortions, although this effect was not significant. Inbreeding depression in total fitness was 29.0%, which corresponds to a difference of 46 seeds per pollinated ovule. Selfing rate estimates were usually intermediate to high, indicating that inbreeding effects observed in this study would be present in naturally pollinated progeny. Although the influence of inbreeding directly on dispersal was negligible, the predicted reduction in sibling competition caused by reduced seed production resulted in an estimate of inbreeding depression of 17.5%, which is 11.5% lower than that measured under uniform conditions. Consequently, inbreeding depression estimated under natural dispersion patterns may be lower than that estimated under uniform conditions since seeds from self- and cross-pollination may not experience the same competitive environment in the field. Inbreeding in the maternal generation, therefore, could influence progeny fitness not only by determining the genetic composition of progeny, but also by influencing the competitive environment in which progeny grow.     

Inbreeding Depression under Drought Stress in the Rare Endemic Echium wildpretii (Boraginaceae) on Tenerife,Canary Islands

How climate-change induced environmental stress may alter the effects of inbreeding in patchy populations of rare species is poorly understood. We investigated the fitness of progeny from experimental self- and cross-pollinations in eight populations of different size of Echium wildpretii, a rare endemic plant of the arid subalpine zone of the Canarian island of Tenerife. As control treatments we used open pollination and autonomous selfing. The seed set of open-pollinated flowers was 55% higher than that of autonomously selfed flowers, showing the importance of animal pollination for reproductive success. The seed set, seed mass and germination rate of seedlings of hand-selfed flowers was similar to that of hand-crossed flowers, indicating weak inbreeding depression (seed set –4.4%, seed mass –4.1%, germination –7.3%). Similarly, under normal watering there were no significant effects of inbreeding on seedling survival (–3.0%). However, under low watering of seedlings inbreeding depression was high (survival –50.2%). Seed set of open- and hand-outcrossed-pollinated flowers was higher in large than in small populations, possibly due to more frequent biparental inbreeding in the latter. However, later measures of progeny fitness were not significantly influenced by population size. We predict that increasing drought duration and frequency due to climate change and reductions of population sizes may increase inbreeding depression in this charismatic plant species and thus threaten its future survival in the longer term.     

Single gene control of postzygotic self-incompatibility in poke milkweed, Asclepias exaltata L

Most individuals of Asclepias exaltata are self-sterile, but all plants lack prezygotic barriers to self-fertilization. To determine whether postzygotic rejection of self-fertilized ovules is due to late-acting self-incompatibility or to extreme, early acting inbreeding depression, we performed three diallel crosses among self-sterile plants related as full-sibs. The full-sibs segregated into four compatibility classes, suggesting that late acting self-incompatibility is controlled by a single gene (S-locus). Crosses between plants sharing one or both alleles at the S-locus are incompatible. An additional diallel cross was done among full-sib progeny from a cross of a self-sterile and a self-fertile plant. These progeny grouped into two compatibility classes, and plants within classes displayed varying levels of self-fertility. This suggests that the occasional self-fertility documented in natural pollinations is caused by pseudo-self-fertility alleles that alter the functioning of the S-locus.     

Pollination biology of Harrisia portoricensis (Cactaceae), an endangered Caribbean species

Traits associated with self-pollination are common features of island plant communities. In this work, we studied the pollination biology and the breeding system of Harrisia portoricensis, an island columnar cactus, to test for the presence of inbreeding and inbreeding depression. For H. portoricensis, which bears flowers with typical outcrossing morphology, the results from 322 h of direct observations and videotaping showed that visits to flowers by animals were uncommon. Controlled pollinations demonstrated that H. portoricensis has a partially self-compatible breeding system that it is not autogamous and thus requires an external mechanism for the movement of pollen to set fruit. We detected differences in seed size, seed mass, germination success, and multiplicative fitness estimates between self- and cross-pollination treatments. We found that progeny resulting from natural and self-pollination treatments showed signs of inbreeding depression compared with progeny resulting from cross-pollination; however, the magnitude of the inbreeding depression was less than 50%. Our combined results suggest that for this species an endogamous breeding system should be favored by natural selection.     

Early-acting inbreeding depression and reproductive success in the highbush blueberry,Vaccinium corymbosum L.

Summary Tetraploid Vaccinium corymbosum genotypes exhibit wide variability in seed set following self- and cross-pollinations. In this paper, a post-zygotic mechanism (seed abortion) under polygenic control is proposed as the basis for fertility differences in this species. A pollen chase experiment indicated that self-pollen tubes fertilize ovules, but are also outcompeted by foreign male gametes in pollen mixtures. Matings among cultivars derived from a pedigree showed a linear decrease in seed number per fruit, and increase in seed abortion, with increasing relatedness among parents. Selfed (S₁) progeny from self-fertile parents were largely self-sterile. At zygotic levels of inbreeding of F>0.3 there was little or no fertility, suggesting that an inbreeding threshold regulates reproductive success in V. corymbosum matings. Individuals below the threshold are facultative selfers, while those above it are obligate outcrossers. Inbreeding also caused a decrease in pollen viability, and reduced female fertility more rapidly than male fertility. These phenomena are discussed in terms of two models of genetic load: (1) mutational load — homozygosity for recessive embryolethal or sub-lethal mutations and (2) segregational load — loss of allelic interactions essential for embryonic vigor. Self-infertility in highbush blueberries is placed in the context of late-acting self-incompatibility versus early-acting inbreeding depression in angiosperms.     

Early-inbreeding depression inVaccinium myrtillus andV. vitis-idaea

Summary Fertility parameters were estimated inVaccinium myrtillus andV. vitis-idaea after self- and cross-pollinations performed in growth chamber. We showed a drastic decrease in fertility after self-pollination as compared to cross-pollination. Number of plump seeds per berry was compared with previous field data. In both species, growth room conditions improved plump seed number after cross-pollination but not after self-pollination. In addition, in order to enhance resources supply to young developing fruits, cytokinin application was tested inV. myrtillus but no effect was detected. We hypothesize that the partial self-sterility is due to inbreeding depression based on the expression of recessive lethals.     

Bee pollination and fruit set of Coffea arabica and C. canephora (Rubiaceae)

Self-sterile Coffea canephora and self-fertile C. arabica are important cash crops in many tropical countries. We examined the relative importance of insect, wind, and spontaneous self-pollination and the degree of self-fertility of these two coffee species in 24 agroforestry coffee fields in Indonesia. In both species, open pollination and cross pollination by hand led to the highest fruit set. Wind pollination (including self-pollination) led to 16% lower fruit set than open pollination in C. canephora and to 12.3% lower fruit set in C. arabica. Self-pollinated flowers and unmanipulated controls achieved an extremely low fruit set of 10% or less in the self-sterile species, and of 60% and 48%, respectively in the self-fertile species. These results constitute experimental evidence that cross pollination by bees causes a significant increase in fruit set of not only the self-sterile, but also the self-fertile coffee species. The practical implication is that coffee yield may be improved by managing fields for increased flower visitation by bees.     

Evolutionary maintenance of stigma-height dimorphism in Narcissus papyraceus (Amaryllidaceae)

Stigma-height dimorphism is a sexual polymorphism in which plant populations are composed of two floral morphs that differ significantly in style length but not anther position. The morphs exhibit approach and reverse herkogamy, floral designs that in most species typically occur as monomorphic conditions. We investigated the floral biology of stigma-height dimorphism in the Mediterranean geophyte Narcissus papyraceus (Amaryllidaceae) in an effort to understand the evolutionary forces maintaining stylar polymorphism. Our survey of 66 populations in Spain, Portugal, and Morocco indicated that 56% were dimorphic with the long-styled morph at an average frequency of 0.79. The remaining 44% of populations sampled were monomorphic for the long-styled morph. In dimorphic populations there was a significant positive relation between population size and the frequency of the short-styled morph. Controlled pollinations demonstrated that N. papyraceus is self-sterile with no significant differences in female fertility between intra- and intermorph crosses. Prior self-pollination reduced seed set in flowers that were subsequently cross-pollinated. Estimates of mating patterns using allozyme markers in eight populations indicated that N. papyraceus is largely outcrossing (mean t(m) = 0.81) with no significant differences between monomorphic and dimorphic populations or style morphs. Stigma-height dimorphism in N. papyraceus is maintained in populations by insect-mediated cross-pollination with biased morph ratios and stylar monomorphism likely resulting from the combined influence of the inheritance of the polymorphism, morph-specific differences in assortative mating and founder effects.     

Is self-compatibility the main breeding system in borage (Borago officinalis L.)?

The polygenic postzygotic self-incompatible system reported in borage (Borago officinalis L.) is refuted. The behaviour of pollen tubes after self- and cross-pollination, analysis of the crossability coefficient, seed set after continuous generations of self-pollination and the effects of inbreeding depression in borage were studied. Evidence of self-compatibility was established. The influence of protandry in a self-compatible system is also discussed. Received: 15 July 1999 / Accepted: 3 November 1999